alturt is pretty much dead, forget about it.
Forget the whole idTech3 engine, forget Unity, forget XreaL, forget UDK, forget CryEngine.
You'd need a _very_ specialized engine, I don't know anything that can handle tons of players and still deliver Quake 3 niveau networking.
Open spaces aren't too much of a problem, even UrT HD will handle them quite well in the end - the biggest problem is networking, and as I said there is probably no engine that is optimized for this.
If you guys get enough artists I'll be in as a programmer